Internal Affairs Minister Joachim Brudzinski said in a statement on Wednesday: "Polish police officers throughout the country have already detained more than 10 since yesterday in connection with online calls for murders and acts of aggression."

"Once again I appeal to stop bad emotions," he said, promising a strong police response.

Police said a 72-year-old Warsaw man was detained on Monday after making a threat during a phone call to a social welfare centre that the next in line to be killed was President Andrzej Duda.

On Wednesday, Polish news agency PAP said police in Gdansk had detained a 48-year-old suspected of making threats on the phone against European Council President and former Polish prime minister Donald Tusk.

Critics have said politicians themselves were cranking up hate speech and aggression in public life, and blamed the ruling Law and Justice party for not reacting earlier.

Mr Adamowicz was one of 11 mayors in Poland who were the targets of fake death certificates by a far-right group called All-Polish Youth after the councillors signed a declaration to welcome refugees, against the Government's anti-migration policies.

The prosecutor's office decided not to pursue that case further.

Vigils and peace rallies held across Poland

Thousands of people gathered in vigils in Polish cities to mourn the Mr Adamowicz's death. He had been mayor of Gdansk for 20 years and espoused liberal causes at odds with the conservative nationalist Government.

Flags flew at half-staff and people signed condolence books at municipal buildings across Poland and in diplomatic missions abroad.

Thousands more marched in anti-hate and anti-violence rallies calling for peace across the country.

Polish authorities arrested a 27-year-old former convict named Stefan in connection with the murder. His full name has so far been withheld.